At least one of the cars began leaking butadiene, which is a colorless, flammable gas, soluble in alcohol but not in water, usually derived from ethanol and butane. It is used chiefly in the manufacture of rubber and paint, and in organic synthesis.

Emergency management declared a Level 3 hazmat, which is the highest hazmat emergency. It forced the evacuation of at more than 30 homes along Abbotts Beach and Katherine Station roads.

Anyone living within a 2½-mile radius of the site was told to shelter in place and not leave their homes in the Code Red situation.

Fire crews said butadiene can explode violently and could produce a health hazard and environmental risks.

The chemical is liquid, has a gaseous quality, a slight sulfur smell and stays low to the ground.

Since the location is so close to the Salt and Ohio rivers, the Coast Guard shut down river traffic.

Just before 9 p.m. the air restrictions, Ohio River restrictions and shelter-in-place orders were lifted.

Dixie Highway was forced to shut down and will remains indefinitely.

The restrictions for 12 homes on Abbotts Beach road are also still in place.

“The greatest risk we have is fire. If this finds an ignition source, the good news is the weather is on our side and with the ignition sources, that's a low risk right now. That’s our greatest risk right now,” said MetroSafe Executive Director Doug Hamilton.

Firefighters used a water mist, a fog pattern to protect vapors, to contain the chemical and block the vapors from getting in the air.

Raw aerials: Crews working at train derailment

Firefighters said the chemical is isolated, but warned that could change at a moment's notice.

"Butadiene is a highly flammable liquid and if it seeks out and finds an ignition source, then the problem actually changes into a different dynamic for us,” said Buechel Fire Department Assistant Chief Rick Harrison.

Firefighters said the tank carrying the chemical has a capacity of 33,000 gallons. The tank was carrying a full load when it derailed. Officials have not said how much leaked.

Officials are also keeping their eyes on a car containing hydrogen fluoride, which would be an inhalation hazard if it begins leaking.

Officials taking precautions after train derailment

Monday's wind blew the vapors out of the area, into a less populated area, which crews called the best-case scenario.

Officials said the nearest resident is six-tenths of a mile from the scene and has been evacuated.

More than 30 homes on Katherine Station and Abbotts Beach roads were evacuated.

Residents on Dixie Beach were asked to evacuate on a voluntary basis.

The area is not densely populated but with the dangers posed by the chemicals, emergency personnel are not allowing residents back to their homes.

The Red Cross has set up a temporary shelter at Muldraugh Elementary School and they're working to determine what the need will be for a long-term shelter.

MetroSafe said the train was heading into Louisville, but couldn’t say from where the train was coming or a cause of the derailment.

Metro government's Code Red system was used twice Monday morning. An initial 1-mile shelter-in-home radius was set early, but was expanded to a 2-mile radius, meaning anyone living in that area should close all windows and doors, close off any leaks and try not to let any outside air get in.

Many motorists were forced to detour off of Dixie Highway as part of the blocked-off radius.

“I wonder what the long-term consequences will be for the citizens who live there, if three is the worst that must have been some dangerous chemicals to drop off,” said Marcus Jones, who was detoured.

“This is not a densely populated area, the area affected, but we do have three counties that are involved, Hardin, Bullitt and Jefferson. We will work with them to identify what long-term shelter we may need,” Hamilton said.

Because crews said they don't have a benchmark other than the Bullitt County train derailment, they're not sure how long these evacuations will be in place. The length of the evacuations depends on when they get the full assessment of the damage.

About 3 p.m., officials determined the butadiene leak appeared to have sealed itself off, which was another positive.

Contractor RJ Corman arrived with various equipment to clean up the derailed cars and inspect the track.

"We're moving a lot quicker through this situation than we had thought originally, which is good news," said Hamilton.

There is no word yet as to how much butadiene leaked. The tank was carrying 30,000 gallons, but officials said a fraction of that went out.

The cleanup at the scene will be handled by a private contractor and take days to complete.

As for why the derailment happened, that's still up in the air and something the National Transportation Safety Board will investigate.